1. Field of the Invention
The present invention relates to a lens barrel and particularly to a lens barrel having a diaphragm mechanism for changing an open F value according to a focal length in a zoom lens (image pickup lens with variable focal length).
2. Description of the Related Art
A zoom lens with a variable focal length in which an open F value is adjusted according to a focal length by limiting a maximum opening amount of a diaphragm is proposed in Japanese Patent No. 3610113. According to this, with a trend of size reduction and higher magnification of a zoom lens, a defocus amount (spherical aberration) tends to get bigger when the diaphragm is opened, and thus, it is proposed that the maximum opening amount of the diaphragm is limited to the extent that the defocus amount does not become poor. That is, an F value when the diaphragm is not opened to the maximum is set as an open F value, and it is so constituted that the diaphragm opening amount is changeable only in a range that a favorable optical performance can be obtained. Moreover, since the defocus amount differs depending on a focal length, it is also suggested that a maximum opening amount to be limited is changed according to a focal length.
However, according to Japanese Patent No. 3610113, since the open F value is limited by limiting on control a maximum opening amount of a diaphragm for exposure control to be controlled by a motor such as an IG meter, there is a problem that diaphragm control gets complicated and delayed. There is also another problem that the diaphragm for exposure control can not be manually operated.
The present invention has been made in view of the above circumstances and has an object to provide a lens barrel which can change the open F value according to the focal length with a simple mechanism and with a high accuracy without deteriorating an optical performance.